In an experiment like that of Avery, MacLeod and McCarty, an extract of heat-killed S cells was…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
In an experiment like that of Avery, MacLeod and McCarty, an extract of heat-killed S cells was split into four parts. One part was left alone as the control. The other three were treated with an enzyme, then mixed with live R cells and spread on plates. The plates were counted for S colonies.
| Plate | Treatment | S colonies |
|---|---|---|
| 1 | none (control) | 180 |
| 2 | protease | 176 |
| 3 | RNase | 182 |
| 4 | DNase | 3 |
- What per cent of the control is the count on plate 2?
- What per cent is plate 3?
- What per cent is plate 4?
- Which molecule carries the transforming principle? Give one reason why plate 4 still shows 3 colonies.
Need a reminder? The key idea
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Control: colonies. Plates 2, 3 and 4: , and .
Plan. Each plate is a share of the control, so divide by and multiply by .
Step 1. (a) .
Step 2. (b) .
Step 3. (c) .
Step 4. (d) Protease and RNase leave the power to transform almost unchanged, and only DNase takes it away. So the carrier is DNA. A few colonies are left because the enzyme does not digest every DNA molecule.
Answer: ; ; ; DNA.
Check. Plates 2 and 3 are close to , and a small difference from the control is chance. Plate 4 is far below.
Watch out. A count above does not mean that RNase helps the transformation. It is a variation of the count.
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